A woman was born with what's known as a phocomelic hand, meaning she was missing a finger and thumb on one of her hands. But when that hand was amputated after a car accident, she developed a five-fingered phantom hand.

We're still not 100% sure what causes the sensation of phantom limbs, even though the vast majority of amputees experience them. The most likely explanation based on the current evidence is that some rudimentary knowledge of what our bodies look like is hardwired into our brains, and phantom limbs are the result of brains trying to reconcile what they think should be there with what's actually there. The brain doesn't seem to come up with an entirely accurate representation of the missing limb — most patients report the phantom limbs are shorter than their real counterparts, and they're usually painful.

One of the most intriguing developments in this field is the recent research with a 57-year-old woman known as RN. Her phocomelic hand had only three working fingers and a rudimentary thumb, yet after her amputation at age 21 she began to experience the sensation of five fingers on her phantom hand. Her brain wasn't relying on what it knew from two decades of direct experience when it simulated the phantom hand — rather, it was pulling from a more basic, intrinsic kind of knowledge. Her brain knew humans usually have five fingers on each hand, even though her rare congenital abnormality meant she had had three.

Even so, her "new" phantom fingers weren't quite the same as her old ones — while, the preexisting fingers felt the same length in phantom form, her thumb and index finger only felt about half their proper length. That's where the story picks up more than three decades later, as RN recently worked with researchers at UC San Diego to extend the sensation of her phantom fingers to the correct lengths. This was accomplished using mirror box therapy, in which her intact right hand was visually duplicated so that she perceived the presence of two normal hands. Talking to New Scientist, researcher Paul McGeoch explains the breakthrough:

"The presence of the deformed hand was suppressing the brain's innate representation of her fingers which is why they appeared shorter, but after the hand was removed and the inhibition taken away, the innate representation kicks in again."